Running count distinct over a column - Oracle SQL - sql

Question

I want to aggregate the DAYS column based on the running distinct counts of CLIENT_ID, but the catch is CLIENT_ID that were seen from the previous DAYS should not be counted. How to do this in Oracle SQL?
Based on the table below (let's call this table DAY_CLIENT):
DAY CLIENT_ID
1 10
1 11
1 12
2 10
2 11
3 10
3 11
3 12
3 13
4 10
I want to get (let's call this table DAY_AGG):
DAYS CNT_CLIENT_ID
1 3
2 3
3 4
4 4
So, in day 1 there are 3 distinct client IDs.
In day 2, there are still 3 because CLIENT_ID 10 & 11 were already found in day 1. In day 3, distinct clients became 4 because CLIENT_ID 13 is not found on previous days.

Answer 1

Here's an alternative solution that may or may not be more performant than the other solutions:
WITH your_table AS (SELECT 1 DAY, 10 CLIENT_ID FROM dual UNION ALL
SELECT 1 DAY, 11 CLIENT_ID FROM dual UNION ALL
SELECT 1 DAY, 12 CLIENT_ID FROM dual UNION ALL
SELECT 2 DAY, 10 CLIENT_ID FROM dual UNION ALL
SELECT 2 DAY, 11 CLIENT_ID FROM dual UNION ALL
SELECT 3 DAY, 10 CLIENT_ID FROM dual UNION ALL
SELECT 3 DAY, 11 CLIENT_ID FROM dual UNION ALL
SELECT 3 DAY, 12 CLIENT_ID FROM dual UNION ALL
SELECT 3 DAY, 13 CLIENT_ID FROM dual UNION ALL
SELECT 4 DAY, 10 CLIENT_ID FROM dual)
SELECT DISTINCT DAY,
COUNT(CASE WHEN rn = 1 THEN client_id END) OVER (ORDER BY DAY) num_distinct_client_ids
FROM (SELECT DAY,
client_id,
row_number() OVER (PARTITION BY client_id ORDER BY DAY) rn
FROM your_table);
DAY NUM_DISTINCT_CLIENT_IDS
---------- -----------------------
1 3
2 3
3 4
4 4
I recommend you test all the solutions against your data to see which one works best for you.

Answer 2

One approach used a correlated subquery:
SELECT DISTINCT
d1.DAYS,
(SELECT COUNT(DISTINCT d2.CLIENT_ID) FROM yourTable d2
WHERE d2.DAYS <= d1.DAYS) AS CNT_CLIENT_ID
FROM yourTable d1
Here is a demo below for SQL Server, but it should also run on your Oracle. I always struggle with setting up Oracle demos.
Demo

Answer 3

You could also use apply operator if oracle support.
select day, CNT_CLIENT_ID
from DAY_CLIENT t cross apply (
select count(distinct CLIENT_ID) as CNT_CLIENT_ID
from DAY_CLIENT
where day <= t.day) tt
group by day, CNT_CLIENT_ID;
In other way use subquery with correlation approach
select day, (select count(distinct CLIENT_ID)
from DAY_CLIENT
where day <= t.day) as DAY_CLIENT
from DAY_CLIENT t
group by day;

Answer 4

Try to keep it simple, always. All other answers also good if you want to learn other ways. But in this case no need to be fancy at all.
SELECT days
, COUNT(DISTINCT client_id) cnt
FROM
(
SELECT 1 days, 10 client_id FROM dual --1
UNION ALL
SELECT 1, 11 FROM dual --2
UNION ALL
SELECT 1, 12 FROM dual --3
UNION ALL
SELECT 1, 11 FROM dual --4
UNION ALL
SELECT 2, 10 FROM dual
UNION ALL
SELECT 2, 11 FROM dual
UNION ALL
SELECT 2, 12 FROM dual
UNION ALL
SELECT 3, 10 FROM dual
UNION ALL
SELECT 3, 11 FROM dual
UNION ALL
SELECT 3, 12 FROM dual
UNION ALL
SELECT 3, 13 FROM dual
UNION ALL
SELECT 4, 10 FROM dual
)
GROUP BY days
ORDER BY 1
/
DAYS | CLIENT_ID
----------------
1 3
2 3
3 4
4 1